Process for electrolytically etching valve metal surfaces



June 22, 1965 J. BURNHAM 3,190,822

PROCESS FOR ELECTROLYTICALLY ETCHING VALVE METAL SURFACES Filed Jan. 9.1961 v 2 sheets-sheet 1 J. BURNHAM 3,190,822

PROCESS FOR ELEGTROLYTICALLY ETCHING VALVE METAL SURFACES June 22, 19652 Sheets-Sheet 2 Filed Jan. 9, 1961 United States Patent O ice 3,190,822p PROCESS FR ELECTRLYTICALLY ETCHING VALVE METAL SURFACES lohn Burnham,10960 Verano Road, West Los Angeles, Calif. Filed Jan. 9, 1961, Ser. No.81,396 Claims. (Cl. 20d-141) This invention pertains to etched surfacesof inert valve metal electrodes and to processes for manufacturing thesame.

In electrolytic capacitors valve metal elect-rodes are used because ofthe asymmetrical electrolytic characteristics of adherent oxide coatedsurfaces of such metals. 4It is established p-ractice to etch suchelectrodes in order to increase their surface areas so as to make itpossible to use relatively small electrodes in order to obtainrelatively high capacitance ratings. The term etch ratio is commonlyused in order to designate the proportionate amount by which theetfectivesurface areas of an electrode is increased by etching.

It is n-ot considered necessary to set forth in this specicaiton t-hereasons why relatively inert valve met-al electrodes, such as tantalum,niobium, and titanium electrodes are used in manufacturing electrolyticcapacitors instead :of electrodes formed of the relatively more reactivemetal aluminum. Unfortunately inert valve metals as indicated abovecannot beetched satisfactorily using the established types of processesfor etching commonly employed with aluminum. As a consequence of this agreat deal of research and development activity has been devoted to theetching of inert valve metals and in particular tantalum.

Such -work has resulted in processes for electrolytic etching tantalumin various types of electrolytes such as, for example, aqueoushydroiluor-ic acid electrolytes and nonaqueous electrolytes in whichsolvents such as methanol or formamide contain inorganic ionogens. Suchprocesses are considered to require critical concentration controlwithin the electrolytes used in them. Frequently such concentrations arerelatively difiicult to achieve and maintain. It isV considered thatprior etching processes of these types vfor use with inert valve metalssuch as tantalum have not been successful in producing etched surfacesof a uniform, satisfactory character having high etch ratios.

An object of this invention is to provide processes for etching inertvalve metals which overcome many of the disadvantages and limitations ofprior processes for the same purposes as briefly indicated in thepreceding discussion. Another object of this invention is to provideetched -surfaces on inert valve metals which are different from theetched surfaces provided on such metals by prior processes and whichhave comparatively high effective sur- Vface areas as indicated by etchratio measurements.

Further objects of this invention are Ito provide processes as indicatedwhich may be easily carried out, which are relatively inexpensive toperform and which may be satisfactorily used with commercially availableinert Valve met-al foils and the like. Related objects of this inventionare to provide etched inert valve metal electrodes which are physicallystrong, which have comparatively high etch ratios and which may beeasily created.

These and other objects of this invention `as well as many specificadvantages of it will be apparent to those skilled in the field ofetching capacitor electrodes from a detailed consideration of theremainder of this specification including the appended claims and theaccompanying drawings in which:

FIG. 1 is a curve indicating the effect of an electrolyte compositionchanges in preparing an etched surface in accordance with thisinvention;

3,190,822 Patented June 22, 1965 FIG. 2 is a photomicrograph of anetched tantalum foil surface created in accordance with this inventionat 200 times its normal dimension; and

t process at 200 times its normal dimension.

A s an faid to understanding this invention it may be summarilyexplained as involving etched, inert valve metal surfaces as indicatedin FIG. 2 of the drawings, these surfaces havinghigh effective areas asopposed to initial surface areas and having .a grooved type ofcomparatively uniform -surface configuration. Such etched foils are, in-accordance with this invention, created by electro-chemically etchingthese surfaces using an elect-rolyte composition containing at least oneadditive which is considered to facilitate the removal of metal so as toachieve a desired type of etched surface configuration Ias indicated inFIG. 2 of the drawings.

Y Although this invention primarily pertains to etched tantalum foilelectrodes such as electrodes created from cold-rolled tantalum foil, iti-s not limited to this type of electrode structure. The inventionwithin its broader scope encompasses inert valve metal electrodes of anydesired shape or configuration formed out of the valve metals tantalum,niobium, and titanium .and various allloys of these metals which arecapable of being used in the s-ame manner as these metals themselves inelectrolytic capacitors as electrodes. Thus, the present inventionencompasses not only foil electrodes but etched electrodes having acup-shaped appearance or etched wire electrodes. With electrodes ofvarious shapes as indicated considerable variations in electrodethicknesses will be encountered. As a general rule, it is preferred toutilize the present in- Vention with cold-rolled metal foils such astantalum foils having a thickness of from about one-half mil t-o aboutone mil.

Any such electrodes have micr-o-crystalline surface structures which:are at least to a limited extent influenced by 4manufacturingprocedures and methods. It is presently lbelieved that many suchelectrodes, such as inert valve metal foils, contain comparativelyminute amounts of surface impurities which tend to affect the ability ofsuch electrodes to -be satisfactorily etched to high etch ratios. Thepresence of these surface impurities is normally detected when effortsare made to form or oxidize these foils to comparatively high voltages.While the precise nature of such impu-rities is not known, it isbelieved that they may consist of various surface oxides and compoundssuch as tantalum carbide, gases such as hydrogen, oxygen or the likelocated within the crystalline structures of electrode surfaces, andthat the concentrations of such impurities will vary depending uponvariations in such crystalline structures. lFurther, the amounts of suchimpurities areipresently considered to be dependent upon not onlymanufacturing methods, but upon conditions to which such surfaces havebeen subjected after their manufacture.

In carrying out this invention an unetched foil of such Yan inert valvemetal may be `first subjected to an initial cleaning stepV if itssurface is not alreadyv substantially free from surface impurities. Thiscleaning step is consideredto remove from its surface some of the valvemetal itself, and more important than this, various surface impuritiesof the type indicated above. In carrying out this cleaning step a valvemetal, such as tantalum foil is made the anode in an electrolytic cellcontaining an inert cathode and a non-aqueous electrolyte which includeseither a solvent or a solvent mixture and a nontilm forming solute orsolute mixture.

Suitable solvent-s for use in such an electrolyte are methanol, ethanol,and the like. Such solvents are of a polar character and are capable ofplacing in solution various types of ionic solutes. If desired, mixturesof such solvents may be employed. A solvent or solvent mixture as hereinspecified may contain trace quantities of various impurities such as upto about 0.10 percent by weight of water on the basis of the weight ofthe solvent without causing any significant effect on the cleaningoperation performed in accordance with this invention. However, thepresence of trace quantities of water in such solvents is considered totend to cause a deeply pitted type of etching to occur during thisinitial cleaning step. A deeply pitted type of etched surface of thegeneral variety referred to in this paragraph is indicated in FIG. 3 ofthe drawings. In accordance with the invention such pitted type etchingis not normally desired. Instead it is preferred to uniformly clean anelectrode surface in order to facilitate the subsequent etching so as toenable the production of the type of etched surface configurationindicated in FIG. 2 of the drawings.

The solutes used in the electrolyte employed for the cleaning stepherein specified should, as indicated in the preceding, be of a non-filmforming ionogen type of and, of course, they should be soluble in thesolvent or solvent mixture employed. Suitable solutes are cornpoundsfurnishing halide ions in the solvents specified. Thus solutes such asammonium chloride, fluoride, bromide, iodide, or bifluoride, or thecorresponding halide salts of lithium, sodium, potassium, ferrie iron,copper, lead, calcium, beryllium, aluminum or the like may be used wheresuch salts are soluble in the solvent or solvent mixture employed. Suchsolubility may be easily determined by reference to appropriatetechnical literature or by routine experimentations. Organic typeionogens such as tetramethyl ammonium bromide, fluoride, chloride oriodide or similar tetra-substituted ammonium halides or mixtures ofthese ionogens and or other ionogens as previously indicated may also beused as solutes in an electrolyte employed during the cleaning stepcarried out in accordance with this invention.

This cleaning step is in the general nature of a polishing step and, asindicated in the preceding, is primarily intended so as to removesurface metal containing impurities and not to accomplish any materialetching of an electrode surface. Because of this the conditions underwhich it may be carried out are primarily of a type wellknown in theelectrolytic field. Thus, the temperature of this initial cleaning stepmay be varied in practice so as to encompass a range of temperaturesfrom about 20 C. to about the boiling point of an electrolyte used. Ingeneral the higher the temperature the more rapidly metal is removedfrom an electrode surface. Satisfactory results can be achieved at roomtemperature.

Similarly the current and voltage used during this cleaning step may bevaried with comparatively wide limits. In general sufficient totalcharge should be used so as to remove substantially all surfaceimpurities. The amount of such total charge will depend upon the amountof such impurities present upon an electrode surface being cleaned orpolished. For this reason it is not considered feasible to set forth inthis specification any range of value indicating this variable incarrying out the process herein described. In practice it is presentlypreferred to carry out cleaning steps as herein specified to a number ofdifferent total current values with electrodes resulting from identicalprocessing or manufacturing conditions, and then to etch the electrodesso cleaned as hereinafter described, and next to measure the etch ratiosof the resultant electrodes in order to determine the amount of cleaningwhich is best used with electrodes resulting from such a singlemanufacturing batch Using this procedure the particular electrode havinga high etch ratio cleaned using less total current than other electrodeshaving substantially the same etch ratio indicates the total amount ofcurrent which should be used during this cleaning operation `or step. Ifan excess of total current is passed through an electrolytic cell duringthe cleaning step, obviously metal is wasted and the strength of anelectrode is decreased unnecessarily. This becomes important when theprocedures set forth herein are used with thin electrode foils such asone-half mil tantalum foils.

The current form used during a cleaning step as herein specified may bevaried over comparatively wide limits. It is preferred to etch with acombined A.C. and D.C. voltage where the R.M.S. value of the A.C.voltage component is approximately equal to the D.C. voltage irnpressed.However, it is possible to utilize during this cleaning or polishingstep square wave form currents and to periodically reverse the polarityin the electrolytic cell employed.

The voltages used during a cleaning or polishing operation as hereindescribed will vary depending upon the specific resistance of theelectrolyte employed as well as the spacing of the anode and the cathodein any cell used. In general, the peak voltages should be held below avalue at which scintillation will occur. Similarly the current densitiesused may be varied within comparatively wide limits. In general thegreater the currentpdensity the greater the amount of metal removed andthe greater the amount of heating within an electrolytic oell.Satisfactory results have been achieved using current densities of fromabout 20 to about 60 milliamperes per square inch of electrode surfaceand voltages consisting of an A.C. half wave superimposed upon a D.C.voltage of equal magnitude to the magnitude of the R.M.S. value of theA.C. wave so that either the D.C. voltage or the R.M.S. value of theA.C. voltage used is from about 2.5 to about volts.

Following the cleaning step indicated in the preceding discussion anelectrode in accordance with this invention is electrochemically etchedas the anode in a cell containing an inert cathode and an electrolytecomposition separating these two electrodes. The preceding cleaning stepmay, of course, be omitted when foils capable of being directly etchedare to be processed. As pointed out in the preceding such foils areconsidered to be substantially free from surface impurities.

The electrolyte composition used during this etching step is consideredto be extremely important in obtaining satisfactory etch ratios.Basically such an electrolyte is the same as the electrolyte as usedduring the cleaning or polishing step previously described except forthe addition of an additive which is capable of facilitating the removalof an inert valve metal from the surface of such a metal during theelectro-etching step. Because of this it is possible to carry out theinvention by adding to an electrolyte as previously described in a celldesired quan titles of one or more additive compounds. It is alsopossible to carry out the process of this invention by physicallytransferring electrodes from one electrolytic cell to another so thatthe cleaning and polishing steps are carried out in separate cellsinstead of in the same cell.

Preferred additives used with the present invention are sulfoxidecomplexing agents which are soluble in anhydrous methanol or similarsolvents, which additions fall within the following general formulaR1-|S-R2 in which the radicals R1 and R2 are of an inert character.Particularly suitable compounds falling within this general formula aredimethyl sulfoxide, diethyl sulfoxide, methylethyl sulfoxide anddiphenyl sulfoxide. Other complexing yagents such as pyridine andsimilar heterocyclic organic compounds containing nitrogen in the ringstructure or substituted derivatives of such compounds in which the ringsubstituted substituents are of an inert character or such as carbondisulfoxide are considered to be capable of being used in achieving atleast some advantages in etching inert valve metals during this step ofthe present invention. A mixture of complexing agents as well as asingle such agent can be employed in the etching electrolyte. Solubilityof such an agent -or agents in the solvent or solvent mixture employedis, of course, required.

The electrolyte used during this etching step may contain comparativelysmall or trace quantities of water. It is presently believed that up toabout 1,000 parts per million of Water on the basis of the total solventor solvent mixture present does not detrimentally affect the characterof theV etch obtained in accordance with this invention. Greaterproportions of the water than this are preferably -avoided in practicingthis invention inasmuch as such greater proportions of this contaminanttend to cause perforation of Ian electrode surface duringelectro-etching, and as a general rule this is to be avoided inachieving the preferred type of etched surface as indicated in FIG. 2 ofthe drawings. Similarly the presence of ketones such as acetone shouldbe avoided in an electrolyte used during this electro-etching step for`substantially the same reasons. Also, various contaminants of this typetend to decrease to a substantial extent the etch ratios which can beachieved in accordance with this invention for various reasons which areat the present time not completely understood or known. Many salt-typecontaminants such as anhydrous magnesium perchlorate also detrimentallyaffect the etching process achieved inaccordance with this invention.

The amount of an additive as herein .specified which 4 this electrolyteare shown on the basis of parts by weight.

From this curve it will be seen that comparatively high etch ratios canbe achieved by the addition of from about 0.5% by weight of thisadditive to about 70% by weight of this additive. The effectiveness ofthe use of an additive of the type indicated over this entire range ising reactions occurring directly upon an electrodesurface, but that itindirectly affects such reactions by tending to facilitate the removalof inert valve metal from such a surface. It is believed that in -allprobability an additive as herein described operates in the process ofthis invention so as to form a solvated complex with a valve metal suchas tantalum which is attacked during the etching operation. It is alsobelieved that such -a complex places such a valve metal in to a formwhere it is removed from the innerfacial areas where electrode reactionstake place, thus facilitating the etching operation. It is also believedthat water interferes with the etching reaction by hydrolyzing thecomplex ion and forming an insoluble hydrated oxide which may coat thesurface and interfere with the flow of current to specific sites on themetal surface. It is presently 'believed that comparatively smalltrace-quantities of such contaminants effectively inhibit or preventthis type of action. This is shown in the point A in FIG. l indicatingan electrolyte having a composition as shown by the curve and containing4.2 parts by Weight of water Ion the basis of the weight of the solventemployed. This Water caused slightly in excess of a drop of etch ratiowhich could be achieved with this invention.

In general the temperature and electrical conditions under which anelectro-etching step as herein specified was carried ,out aresubstantially the same as the conditions used in carrying out anelectro-cleaning step as vary the etch ratio obtained in a finalelectrode.

previously specified. For this reason it is not considered necessary torepeat the preceding discussion as to temperature, voltages, currentsand the like. The amount of total current used during the etching will,of course, affect the amount of etching achieved and, hence, will As ageneral rule it is desired to carry out the electro-etching so as toachieve as high an etch ratio as possible and to use no more currentthan is necessary in order to achieve such an etch ratio since etchingbeyond a maximum etch ratio only serves to remove metal and, hence,serves to weaken the mechanical structure of an electrode. The amount oftotal current which should be used in etching will vary somewhatdepending upon the final results achieved with this invention. Ingeneral, however, it can be stated that satisfactory etch ratios can beachieved by passing from about to about 200 coulombs of current persquare inch of electrode surface etched.

It will, of course, be realized that the final etch ratio achieved onany electrode surface is normally determined by means of capacitancemeasurements taken after such a surface is anodized in accordance withconventional practice. Depending upon the conditions of such anodizationthe etch ratio which is measured for any electrode will vary overcomparatively wide limits. Thus, for example, cold rolled tantalum foilswhich have been cleaned and etched in an identical manner in accordancewith this invention have been determined to have an etch ratio of about14 when formed in accordance with con-l ventional practice at 25 voltsand of about 4 when formed at volts under identical conditions except asto voltage.

From an examination of FIG. 2 of the drawings showing an etched surfacecreated in accordance with this invention it will be realized that theseetched surfaces as herein described are of a comparatively uniformlyetched character, and that when viewed under a microscope they have theessential appearance of a series of ridges and valleys, these ridges andvalleys being substantially regularly located parallel to one anotherand all having an uneven surface configuration. This regularity and thedistance between successive ridges is determined by the fineness of themetallographic grain yand the direction of the Valleys and ridges isdetermined by the direction of rolling of the metal. This results fromthe fact that the rolling operation breaks down crystals into smallerones and the fragments are laid out along lines parallel to the rollingdirection. The fineness of the grain is of particular importancetherefore to the ratios of surface developed on etching and metal havingthe maximum degree of cold working by rolling, extruding or drawing ismuch to be preferred.

This type of etched surface is considered to be differ- -ent in kindfrom etched surfaces of the general Variety indicated in FIG. 3 of thedrawings created in accordance with known techniques. Such etchedsurfaces on tantalum and other inert valve metals are of essentially apook-marked configuration and they appear to be essentially smoothexcept where they are intersected by what may be termed holes extendinginto the metal surface. The smooth areas in prior art electrodes asindicated in FIG. 3 are considered'to be areas where no significantincrease in effective surface area has been achieved by the etchingprocess used in creating them. The smooth areas seen on the prior artelectrode surface represent areas which have been essentially leftuntouched by the electrochemical reaction and it is believed that thesemay be caused by very fine films of compounds of tantalum which areresistant to etching and that with prior processes the etching actionachieved primarily follows boundary lines between crystalline regions inthe foil being treated. The reduced etch ratios obtained in the priorart thus appear to be due to the fact that not all the metal surfacecornes into play. The amount of tantalum dissolved in forming theelectrode surfaces shown in FIGS. 2 and 3 was exactly the same. Anotheradvantage of the present invention is that it allows very thin foils, aslow as 0.2 mil, to be etched successfully Without perforation.

If desired, it is possible to utilize the procedures as herein specifiedin conjunction with various types of known etching steps for inert valvemetals such as tantalum so as to etch metal surfaces which havepreviously been etched so as to have surface congurations of the typeindicated 4in FIG. 3 of the drawings. When this is done the beneficialresults of this invention -appareutly not only affect flat surface areasas indicated in FIG. 3 but affect an increase of effective surface areaswithin the pock-marked regions indicated in this figure. If desired,such electrodes can be completely perforated by prior etching processesand then treated in accordance with this invention. Such perforated andetched electrodes then can be rolled into a type of capacitor anodehaving characteristics which are similar to the characteristics of theso-called sintered type of capacitor anodes made from small particles ofvalve metals. It is not generally preferred, however, to utilize thepresent invention with prior types of etching processes serving toperforate or deeply pitmetal surfaces because of the effect of suchperforations or pits when further treated in accordance with thisinvention to lower the mechanical strength of the electrode member.

As an aid to understanding this invention the preceding specificexamples of carrying out processes as herein described in creatingetched surfaces of the type indicated in FIG. 2 of the drawings aregiven. These examples are given by way of illustration only and are notconsidered as limiting this invention. For convenience of presentationthese examples are -set forth in tabular form. All of the examplespertain to the treatment of commercially available one-half inchcold-rolled tantalum foil. In all of these examples identicalelectrochemical cells were employed.

In all examples the etch ratioswere determined after forming (oxidizing)the etched electrodes indicated to 25 volts at an electrolytetemperature of about 90 C. using 20 milliamp. current. The same currentwas used in all of the forming operations regardless of electrode sizeor the degree to which an electrode had been etched. In the forming theelectrodes in Examples I to XII the electrolyte employed contained: 60parts by weight ethylene glycol; 40 parts by weight water; and 10 partsby weight oxalic acid. In all other examples the electrolyte contained lpart by weight ammonium phosphate and 1000 parts by weight water.

I claim: v

1. A process for electrolytically etching an inert valve metal selectedfrom the group consisting of tantalum, niobium and titanium whichcomprises:

electrochemically etching an inert valve metal member comprising aninert valve metal selected from the group consisting of tantalum,niobium and titanium in an electrolyte including a non-aqueous solventand an ionogen solute dissolved in said solvent, said solute providinghalogen radicals in said electrolyte, said electrolyte also including anadditive having the general formula R1g-Rs in which R1 and R2 are loweralkyl or aryl radicals, said additive being soluble in said electrolyte,said additive being Vcapable of forming a solvated complex with ions ofsaid inert valve metal removed from said inert valve metal member duringthe electrochemical etching of said inert valve metal mem ber, saidelectrolyte being substantially ketone free and containing no more than1,000 ppm. of water,

said additive being present in an amount suiicient to form such acomplex.

2. A process for electrolytically etching an inert valve metal selectedfrom the group consisting of tantalum, niobium and titanium whichcomprises:

electrochemically etching an inert valve metal member comprising aninert valve metal selected from the group consisting of tantalum,niobium and titanium in an electrolyte including a non-aqueous solventand an ionogen solute dissolved in said solvent, said solute providinghalogen radicals in said electrolyte, said electrolyte also including anadditive selected from the group consisting of dimethyl sulfoxide,diethyl sulfoxide, methylethyl sulfoxide and diphenyl sulfoxide, saidadditive being capable of forming a solvated complex with ions of saidinert valve metal removed from said inert valve metal member during theelectrochemical etching of said valve metal member in said electrolyte,said electrolyte being substantially ketone Ifree and containing no morethan 1,000 p.p.m. of water, said additive being present in an amountsuiiicient to form such a complex.

3. A process for electrolytically etching an inert valve metal selectedfrom the group consisting of tantalum, niobium and titanium whichcomprises:

electrochemically etching an inert valve metal member .comprising aninert valve metal selected Vfrom the group consisting of tantalum,niobium and titanium in an electrolyte including a non-aqueous solventand an ionogen solute dissolved in said solvent, said solute providinghalogen radicals in said electrolyte, said electrolyte also including aheterocyclic saturated organic compound of the pyridine family, saidadditive being soluble in said electrolyte, said compound serving as anadditive capable of forming a solvated complex with ions of said inertvalve metal removed from said inert valve metal member during theelectrochemical etching of said valve metal member in said electrolyte,said electrolyte being substantially ketone free and containing no morethan 1,000

p.p.m. of Water, said additive being present in anV amount suflicient toform such a complex.

4. A process for electrolytically etching an inert valve metal selectedfrom the group consisting of ltantalum, niobium and titanium whichcomprises:

electrochemically etching an inert valve metal member comprising aninert valve metal selected from the group consisting of tantalum,niobium and titanium in an electrolyte including a non-aqueous solventand an ionogen solute dissolved in said solvent, said solute providinghalogen radicals in said electrolyte, said electrolyte also includingpyridine, said pyridine serving as an additive capable of forming asolvated complex with ions of said inert valve metal removed from saidinert valve metal member during the electrochemical etching of saidvalve metal member in said electrolyte, said electrolyte beingsubstantially ketone free and containing no more than 1,000 p.p.m. ofwater, said additive being present in an amount suiiicient to form sucha complex.

5. A process for electrolytically etching an inert valve metal selectedfrom the group consisting of tantalum, niobium and titanium whichcomprises:

electrochemically etching an inert valve metal member comprising aninert valve metal selected from the group consisting of tantalum,niobium and titanium in an electrolyte including a non-aqueous solventand an ionogen solute dissolved in said, solvent, said solute providinghalogen radicals in said electrolyte, said electrolyte also includingcarbon disulfide, said caribon disulfide serving as an additive capableof forming a solvated complex with ions of said inert valve metalremoved from said inert valve metal member during the electrochemicaletching of said valve metal member in said electrolyte, saidelectroinert valve metal electrode member comprising an inert valvemetal selected from the group consisting of tantalum, niobium andtitanium in a first electrolyte, said first electrolyte consisting of anon- 6. A process for electrolytically etching an inert valve 5 metalselected from the group consisting of tantaluni, niobium and titaniumwhich comprises:

electrolytically cleaning surface impurities from an aqueous solventcontaining in solution a non-film forming ionogen capable of providingin said first electrolyte a halogen radical; and

electrochemically etching said member in a second elecinert valve metalelectrode member comprising an said additive being soluble in saidelectrolyte, said additive being capable of forming a solvated compleXwith ions of said inert valve metal removed from said inert valve metalmember during the electrochemical etching of said inert valve metalmember in said second electrolyte, said second electrolyte beingsubstantially ketone free and containing no more than 1,000 p.p.m. ofwater, said additive being present in an amount sufficient to form sucha complex.

trolyte, said second electrolyte including a non-aqueinert valve metalselected from the group consisting ous solvent and an ionogen solutedissolved in said of tantalum, niobium and titanium in a first elec-Solvent, said solute providing halogen radicals in trolyte, said firstelectrolyte consisting of a nonsaid second electrolyte, said secondelectrolyte also aqueous solvent containing in solution a non-filmincluding a heterocyclic saturated organic compound forming ionogencapable of providing in said first of the pyridine family, said additivebeing soluble electrolyte a halogen radical; and in said electrolyteserving as an additive, said addielectrochemically etching said memberin a second elective being capable of forming a solvated complextrolyte, said second electrolyte including a non-aquewith ions of saidinert valve metal removed from ous solvent and an ionogen solutedissolved in said Said inert valve metal member during theelectrosolvent, said solute providing halogen radicals in said chemicaletching of said inert valve metal member second electrolyte, said secondelectrolyte also inin Said Second electrolyte, said second electrolytecluding an additive having the general lformula being substantiallyketone free and containing no O more than 1,000 p.p.m. of Water, saidadditive being 1| present in an amount sufiicient to form such a com-R1`S-R pleX. lll Wlllcll R1 and R2 are lower alkyl 0f aryl radicals, 259. A process as defined in claim 8 wherein said heterocyclic organiccompound is pyridine, said additive being present in an amountsuiiicient to form such a complex. 10. A process for electrolyticallyetching an inert valve metal selected from the group consisting oftantalum, niobium and titanium which comprises:

electrolytically cleaning surface impurities from an inert valve metalelectrode member comprising an inert valve metal selected from the groupconsisting of tantalum, niobium and titanium in a first electrolyte,said first electrolyte consisting of a non- 7. A process forelectrolytically etching an inert valve metal selected from the groupconsisting of tantalum, niobium and titanium which comprises:

electrolytically cleaning surface impurities from an inert valve metalelectrode member 'comprising an 40 inert valve metal selected from thegroup consisting of tantalum, niobium and titanium in a firstelecaqueous solvent containing in solution a non-film forming ionogencapable of providing in said first electrolyte a halogen radical; and

electrochemically etching said member in a second electrolyte, saidsecond electrolyte including a non-aqueous solvent and an ionogen solutedissolved in said solvent, said solute providing halogen radicals insaid trolyte, said first electrolyte consisting of a nonaqueous solventcontaining in solution a non-lm forming ionogen capable of providing insaid first second electrolyte, said electrolyte also including carbondisulfide serving as an additive, said additive being capable of forminga solvated complex with electrolyte a halogen radical; and

electrochemically etching an inert valve metal member comprising aninert valve metal selected from the group consisting of tantalum,niobium and titanium in a second electrolyte including a non-aqueoussolvent and an ionogen solute dissolved in said solvent,

ions of said inert valve metal member during the electrochemical etchingof said inert valve metal member in said second electrolyte, saidelectrolyte being substantially ketone free and containing no more than1,000 p.p.m. of water, said additive being present in an amountsufficient to form such a cornsaid solute providing halogen radicals insaid second p1eX electrolyte, said second electrolyte also including anadditive selected from the group consisting of di- References Citedbythe Examiner methyl sulfox'ide, diethyl sulioxide, methyl ethyl sulf-55 UNITED STATES PATENTS oxide and diphenyl sulfoxide, said additivebeing capable of forming a solvated complex with ions 4431170 5/48 Smlth29-180 of said inert valve metal removed from said inert 216991599 l/Potcllell 29-180 valve metal member during the electrochemical etch-2:742416 4/56 Jenny 204--141 ing of said valve metal member in saidsecond elec- 217751553 12/56 Kahan 204-l4l trolyte, said secondelectrolyte being substantially 2186?8ll 12/58 Rusetla 204-141 ketonefree and containing no more than 1,000 10701522 12/62 Robll'lson et al204-141 ppm. of water, said additive being present in an FOREIGN PATENTS1 amount sufficient to form such a compex. 593,403 3/60 Canada.

8. A process for electrolytically etching an inert valve metal selectedfrom the group consisting of tantalum, niobium and titanium whichcomprises:

electrolytically cleaning surface impurities from an WINSTON A. DOUGLAS,Primary Examiner. JOHN H. MACK, JOHN R. SPECK, Examiners.

1. A PROCESS FOR ELECTROLYTICALLY ETCHING AN INERT VALVE METAL SELECTEDFROM THE GROUP CONSISTING OF TANTALUM, NIOBIUM AND TITANIUM WHICHCOMPRISES: ELECTROCHEMICALLY ETCHING AN INERT VALVE METAL MEMBERCOMPRISING AN INERT VALVE METAL SELECTED FROM THE GROUP CONSISTING OFTANTALUM, NIOBIUM AND TITANIUM IN AN ELECTROLYTE INCLUDING A NON-AQUEOUSSOLVENT AND AN IONOGEN SOLUTE DISSOLVED IN SAID SOLVENT, SAID SOLUTEPROVIDING HALOGEN RADICALS IN SAID ELECTROLYTE, SAID ELECTROLYTE ALSOINCLUDING AN ADDITIVE HAVING THE GENERAL FORMULA